The present invention is primarily but not exclusively concerned with drilling of holes for receiving explosives in a blast hole operation. One of the primary objectives of blast hole drilling is to produce straight holes and to increase drill bit life. For uniform breaking of a rock mass in the resulting explosion, it is important to have blast holes that are straight and accurately positioned. Production of straight holes depends on the feed force, rotation speed and torque applied through a "down the hole" (DTH) "in the hole" (ITH) percussion hammer to a drill bit. These drilling parameters are set according to the geological conditions of the rock mass.